An ink-jet recording apparatus which records an image by using an ink-jet recording head (hereinafter referred to as a recording head) which discharges micro ink droplets from a nozzle discharges the ink droplets from the nozzle by giving a pressure through operation of pressure generating unit to ink in a pressure chamber and has them land on a recording medium such as recording sheets. As the pressure generating unit, a piezoelectric material such as PZT which is electric/mechanical converting unit is used in general. The piezoelectric material is sandwiched by two driving electrodes and is subjected to deformation driving by having a driving waveform with a predetermined voltage applied between these driving electrodes, and this deformation driving expands/contracts the capacity in the pressure chamber so as to give a pressure to the ink in the pressure chamber for discharge.
Such ink-jet recording method is capable of highly accurate image recording with a relatively simple configuration and has been rapidly developed in a wide variety of fields from household to industry. Particularly for higher speed and higher image qualities, various improvements have been proposed, and there is a high demand for high-speed printing by using a recording head such as one-pass printing using a line head and the like, while there is also a demand for higher image quality by improving gradation of printed images.
Also, JP-A-2011-5815 or JP-A-2001-205826 discloses technologies relating to the gradation include an ink-jet recording apparatus which realizes multi-gradation by selecting dots having different droplet sizes in every pixel cycle
In JP-A-2011-5815, as illustrated in FIG. 13, dedicated driving waveforms for the respective droplet sizes are prepared and used in accordance with the desired droplet size to be discharged within 1 pixel cycle T. (a) in the figure illustrates a driving waveform used when small dots are to be discharged, (b) is a driving waveform used when medium dots are to be discharged, and (c) is a driving waveform when large dots are to be discharged.
In JP-A-2001-205826, as illustrated in FIG. 14, for example, a driving waveform in which a plurality of types of waveforms having shapes different from each other are sequentially generated in a predetermined order for each pixel cycle T is prepared, and a portion to be used at discharge (portion where a switch circuit is turned on) in the driving waveform in which the plurality of types of the waveforms are juxtaposed is selected in accordance with the desired droplet size to be discharged so that dots having different sizes can be printed individually. For example, (a) in the figure illustrates the entire driving waveform generated in 1 pixel cycle T (sections Ta to Tf), (b) forms a small dot by turning on only the waveform portions in the sections Ta and Te among them, (c) forms medium dots by turning on only the waveform portion in the section Tc, and (d) forms large dots by turning on only the waveform portion in the section Tf.
JP-A-2011-5815 has a problem that a burden on a driving circuit is large since the dedicated driving waveforms which are different depending on the droplet size are individually needed.
On the other hand, in JP-A-2001-205826, the common driving waveform illustrated in FIG. 14(a) can be used for different droplet sizes, but when an ink droplet is to be discharged actually, only a portion in the entire driving waveform is used in 1 pixel cycle T. Thus, time for the unused waveform portion is wasted, and the driving cycle is prolonged for that portion, which is a serious problem in high-speed printing.